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GENES AND CHROMOSOMES

An understanding of basic cytogenetics is required before proceeding into specific cockatiel mutation crosses and the expected outcome. Cytogenetics is the science which attempts to correlate cellular events with genetic phenomena.

Genes are made up of DNA and in simple terms one or more genes produce an expressed trait or phenotype. In addition, a single gene can effect several processes and in turn produce several traits. This multiple effect is termed pleiotropism. In the process of mutation(s), a gene may be changed into two or more alternative forms called alleles. The term allele and gene are used interchangeably. A trait, either whole or in part, is produced by a pair of genes, one received from the male parent and one from the female parent. (The exception is sex-linked genes where the trait is expressed in female birds with the gene only present on the single X sex chromosome.) If both genes are identical, then this genetic state is termed homozygous. If the pair of genes are different then this is termed heterozygous or hybrid. The gene that can express itself in the homozygous, as well as, heterozygous condition is referred to as the dominant factor. The gene that can only express itself in the homozygous condition is referred to as the recessive factor.

When dealing with a trait that has a dominant and recessive gene, there are three common conditions. The first is homozygous dominant in which both genes present are the dominant ones. The second is homozygous recessive in which both genes present are the recessive ones. The third is heterozygous in which both the dominant and recessive genes are present. Breeders often refer to the heterozygous condition as “split”.

There is also a condition which exists for some genes called codominance. In this case, each gene is capable of some degree of expression in the heterozygous condition. Other terms synonymous with codominance are partial dominance, semidominance, incomplete dominance and additive genes.

Keep in mind that not all traits are controlled by a single pair of genes. A trait can be controlled by numerous genes on the magnitude of 100 or more. Traits that are controlled by numerous genes are termed quantitative traits.

The genetic DNA in conjunction with a protein matrix forms nucleoprotein and becomes organized into structures called chromosomes. Chromosomes are located within the nucleus of a cell. In summary, genes reside on chromosomes.

Figure 1 shows the basic anatomy of a chromosome.


The chromosomes within a non-replicating cell are strung out as fine filaments. It is when a cell is involved in cellular division that the chromosomes take on a condensed “X” appearance as shown in Figure 1. The two chromatids on the same chromosome are referred to as sister chromatids. Sister chromatids are the result of replication of genetic material and normally, they are a photocopy of each other. There are genetic events which could make them dissimilar. I will explain those events at a later time. Chromomeres are regions of dense DNA. When stained, they appear as dark regions under the microscope. The location of the centromere will vary depending upon the chromosome. If the centromere is in the center it is referred to as metacentric, if off center, submetacentric or acrocentric, if very near one end, telocentric.

Chromosomes are uniquely identified by their banding pattern when stained. There are several differential banding procedures - Giemsa (G) Banding, C Banding, R Banding and NOR Banding. Each method can be easily employed on macrochromosomes (chromosomes greater than 1 µm in length), unfortunately birds possess minute microchromosomes (chromosomes 1 µm or less in length). These microchromosomes are barely visible under the light microscope and thus difficult to identify even if differential banding is performed.

Figure 2 is a picture I took a while back. It shows Giemsa banding on a human lymphoma-Armenian hamster immunoblast hybridized somatic cell. A somatic cell is any body cell other than a sexual reproductive cell. As you can see with these macrochromosomes, some show distinctive bands and not all of the chromosomes are identifiable.

The exact number of chromosomes (karyotype) possessed by a cockatiel is unknown. Avian karyotypes can range upwards to 40 - 80 chromosomes.

There are two categories of chromosomes, autosomes and sex chromosomes. With cockatiels, sex is determined by a heteromorphic (i.e. morphologically dissimilar) pair of chromosomes called sex chromosomes. In humans and some other species, these chromosomes are labeled X and Y. A male human has the XY complement (the heterogametic sex) and a female human has the XX complement (the homogametic sex). In some species, such as birds, the complement of sex chromosomes is the opposite of the above. Geneticists have assigned a different lettering scheme to make this distinction. The letters used are Z and W. A male cockatiel has the complement ZZ (the homogametic sex) while the female has ZW (the heterogametic sex). However, you will find that nearly every avian breeding presentation will use the X and Y designation for sex chromosomes in birds. For the remainder of this presentation, I will use XX to describe the male cockatiel sex chromosomes and XY for the female. This is the only deviation I will take from established genetic doctrine in order to prevent any unnecessary confusion with the reader. All chromosomes exclusive of the sex chromosomes are called autosomes. Autosomes occur in morphologically similar pairs referred to as homologues.

Realize that the Y chromosome in cockatiels is most likely void of any genes. In some species of animals, XX (two X chromosomes) is a male and XO (just one X, no Y chromosome) is a female. The O symbolizes lack of a sex chromosome. Therefore, one could reasonably conclude that the XO condition in cockatiels would produce a male. Such events do occur among various species in nature, including humans at a ratio of 1:5000 (Turner syndrome).



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